This invention relates to a defrost operation system and, more particularly, to an air conditioner to which a hot gas bypass defrost system is adopted so as to effect defrost while blowing warm air into a room.
A conventional defrost system for use in an air conditioner, which has a heat pump of a refrigerant circuit formed by successively connecting, via pipings, a compressor, a four-way valve, an indoor heat exchanger, an expansion valve, an outdoor heat exchanger and other components, effects cooling and heating modes of operation by switching over the four-way valve. Since, in this conventional system, defrosting is effected after adapting the refrigerant circuit for the cooling modes of operation by switching over this circuit from a state corresponding to the heating mode of operation to that corresponding to the cooling mode of operation, cooled air is necessarily blown into the room. One possible measure for reducing the flow of cooled air as small as possible is an air heater juxtaposed with the indoor heat exchanger.
Another type of defrost system has been known as a hot gas bypass defrost system. This defrost system operates to melt layers of frost which have sticked to or accumulated on the indoor heat exchanger during heating mode of operation by directly bypassing discharge gas (hot gas) to the outdoor heat exchanger. In this system, the rate of back liquid to the compressor is large because the refrigerant after defrost is directly drawn into the compressor and because this system lacks an evaporator for causing the refrigerant to evaporate after defrost.
A type of countermeasure to this problem was proposed in U.S. Pat. Nos. 4,279,129, 4,215,555 and 4,286,435. In a hot gas bypass defrost system in accordance with these patents, hot gas is bypassed to the outdoor heat exchanger and also to an accumulator on the inlet side of the compressor, but no closing valve is disposed in the bypassing pipe of the accumulator, and the hot gas is controlled by the temperature of refrigerant at the inlet thereof. Therefore, there is a possibility of frost accumulated on the outdoor heat exchanger to be melted excessively rapidly. For this reason, there is a problem that a sensor disposed on the inlet pipe of the compressor may be inoperative (constantly at a saturation temperature) or that the defrost time may be increased because it is necessary to reduce the flow rate of bypassed hot gas when the degree of superheating of discharged gas, is enhanced high enough to make the sensor operative.